Zinc oxide is a well known component of coatings for automotive and architectural glazings. Such coatings are useful for providing enhanced performance characteristics, e.g., anti-reflectivity, electrical resistivity, infrared energy rejection, etc. A zinc oxide layer may conveniently be deposited onto a transparent substrate, either singularly or in combination with other metal and dielectric layers, by conventional methods such as, for example, spray pyrolysis, sputtering, vacuum evaporation, or chemical vapor deposition.
A zinc oxide layer which is particularly useful for preparing high performance automotive and architectural glazings is formed by the spray pyrolysis process, in which a reactant mixture containing a soluble zinc salt is sprayed onto a hot glass ribbon being produced by the float glass process. The zinc-containing compound pyrolyzes at the surface of the glass and, in the presence of oxygen, produces a layer of zinc oxide thereon. Several factors are important in determining the thickness and uniformity of the resultant zinc oxide layer. The temperature of the substrate must be high enough to effect the pyrolysis reaction, generally at least about 900.degree. F. for the deposition of zinc oxide. The reactant mixture may be diluted with organic solvents or water for ease of control. However, the degree of dilution is limited by the need to attain a useful film deposition rate, and also by the chemistry of the pyrolysis reaction itself since the solubility limit must not be approached under the solution in-flight conditions. The spray pattern and velocity distribution of the droplets, as well as the turbulence and lateral vapor movement at the surface of the substrate, likewise affect the quality of the deposited layer. Given a particular zinc-containing pyrolytic precursor, the above enumerated process factors may be adjusted by routine experimentation to optimize the zinc oxide layer so produced.
A particularly useful and well-known zinc-containing pyrolytic precursor is zinc acetate. See, for example, Major et al., "Highly Transparent and Conducting Indium-Doped Zinc Oxide Films by Spray Pyrolysis," Thin Solid Films, 108 (1983) 333-340. Although other zinc-containing pyrolytic reactant mixtures yield higher deposition rates and therefore thicker zinc oxide layers under the identical process conditions, spray pyrolysis utilizing zinc acetate results in a higher quality film having a much more uniform appearance, higher optical transmittance, and greater electrical resistivity. Zinc oxide layers produced utilizing zinc acetate comprise columnar crystallites with average diameters of about one Angstrom and heights equal to the film thickness. The prior art does not suggest that spray solution adjuvants may be useful to accelerate the growth rate for zinc oxide layers deposited by a spray pyrolysis process utilizing zinc acetate as the zinc-containing pyrolysis reactant mixture precursor.
U.S. Pat. No. 3,850,665 to Plumat et al. discloses a process for forming a metal oxide coating on a substrate, wherein an acetyl acetonate coprecipitate of two or more metals such as, for example, zinc and cobalt is dissolved in a solvent and sprayed onto a hot glass ribbon. The resultant coating is a metal oxide containing zinc cobaltite (ZnCo.sub.2 O.sub.4). The patent does not teach the addition of an acetyl acetonate to a zinc acetate-containing solution for spray pyrolysis, for increasing the growth rate of a zinc oxide layer.
U.S. Pat. No. 4,292,347 to Donley discloses a pyrolytic coating reactant for producing durable films, comprising a metal carboxylate such as, for example, zinc 2-ethyl hexonate and a diketonate such as, for example, cobaltous acetyl acetonate. The patent states that the combination of these pyrolytic coating reactants improves the durability of the produced coating, but does not substantially alter the reactivity of the coating composition nor the growth rate of the deposited layer.
It would be desirable to increase the growth rate of a zinc oxide layer deposited onto a hot glass ribbon utilizing a spray pyrolysis process, wherein zinc acetate is the zinc-containing pyrolytic precursor. Zinc oxide layers produced utilizing zinc acetate as the pyrolytic precursor have superior optical and physical properties, but suffer from lower growth rates than when other zinc-containing pyrolytic precursors are used.